This invention relates to a double wall plastic article formed with a pair of thermoplastic sheets and a method and apparatus for differential pressure forming a plastic article with a pair of deformable plastic sheets.
Double web, differential forming apparatus has been provided heretofore such as that disclosed in U.S. Pat. No. 3,583,036, granted to G. W. Brown on June 8, 1971. Such apparatus, however, is used only to form a single wall thickness vessel.
To increase the insulating characteristics of an article formed from a single plastic sheet, the sheet thickness can be increased, however, the use of higher density, thicker, single sheet plastic stock is relatively expensive. Accordingly, it is another object of the present invention to provide a plastic article having increased strength for reduced material cost.
It has been found that a double wall vessel has improved insulating characteristics. Moreover, for a given amount of material and a predetermined wall thickness, a double wall, especially one having walls separated by a gap, will exhibit increased physical strength and improved insulating characteristics as opposed to a single wall.
Accordingly, an object of the present invention is to provide a double wall plastic article which is aesthetically pleasing.
Another object of the present invention is to provide a double wall object and a method and apparatus for forming a double wall object having increased strength but utilizing the same amount of material.
The corrugation of one of the walls of the double wall article will result in increased wall strength. It is desirable that only the inner wall be corrugated so that the corrugations or ribs formed in the wall not be readily visible to a consumer purchasing a packaged product. With a single wall construction, when the inner wall surface is deformed to a particular shape, the outer wall surface will generally assume the same contour as the contour of the inner wall surface. Accordingly, it is an object of the present invention to provide a double wall plastic article having one wall thermoformed to a shape which does not follow the contour of the other wall.
A further object of the present invention to provide an article with a double layer wall including corrugations in one of the layers and a smooth outer exposed surface in the other of the layers.
It is another object of the present invention to provide an article of the type described including inner and outer walls, one of the walls having a plurality of perimetrically extending ribs and a plurality of perimetrically spaced, radially extending ribs joined to the perimetrically extending ribs but having different radial lengths.
It is another object of the present invention to provide a plastic container having upper and lower container portions which are releasably held together by a new and novel latch embodied in one layer of a double layer wall of one of the container portions.
Another object of the present invention is to provide a container having double layer wall including a latch receiving portion formed in only one of the layers.
Apparatus, such as that disclosed in U.S. Pat. No. 3,664,791 granted to G. W. Brown on May 23, 1972, has been provided heretofore for successively delivering a heated plastic sheet to a sheet heating station and then to a forming station. The Brown U.S. Pat. No. 3,664,791 discloses a single pair of laterally spaced endless chains which carry the sheet of plastic. The system disclosed in U.S. Pat. No. 3,664,791 has been previously used in twin sheet thermoforming by mounting a second plastic sheet on a second set of identical endless chains in vertically spaced relation with the set of chains illustrated in the drawings of U.S. Pat. No. 3,664,791. The opposed molds are then brought into engagement with the vertically spaced sheets to form single wall thickness, upper and lower opposed container halves which, after being formed, are then moved together into engagement to form an article similar to that illustrated in FIG. 3 of the Brown U.S. Pat. No. 3,583,036. One problem attendant with the prior art apparatus is that substantial scrap material results because the thermoplastic sheets used in the prior art apparatus must be substantially wider than the article to be formed to accommodate the vertical movement of the sheets between the vertically spaced positions and the adjacent sheet engaging positions. Moreover, the two sets of vertical chains are costly to build and maintain.
The known prior art, twin sheet thermoforming device generally includes air blowing apparatus for maintaining the thermoplastic sheets spaced apart, before forming and during forming, until the opposed container parts are formed. Such equipment is expensive to maintain. Accordingly, it is an object of the present invention to provide method and apparatus for forming a double wall article in a pair of contiguous thermoplastic sheets supported by a single set of laterally spaced, longitudinally extending rails.
It is another object of the present invention to provide a method and apparatus for forming a double wall article which will minimize the scrap material.
It is another object of the present invention to provide a method and apparatus of the type described which eliminates the air injection blow apparatus used heretofore to separate the sheets prior to forming.
Another object of the present invention is to provide a method and apparatus of the type described which compresses thermoplastic sheets together to fuze opposed portions of the sheets and then applying differential pressure to a central portion of at least one of the sheets within the fuzed sheet portions to withdraw the central sheet portion to complete the shape of the article.
Another object of the present invention is to provide method and apparatus of the type described for forming a double wall thickness plastic article, in a pair of thermoplastic sheets including method and apparatus for sealing opposed, border portions of the sheets and then drawing a central portion of one of the sheets away from the opposed central portion of the other sheet.
The method and apparatus according to the present invention mounts a pair of polystyrene foam sheets, such as oriented polystyrene, foam polystyrene or high impact polystyrene, in face-to-face contiguous relation. The sheets pass to a heating station and are heated to forming temperatures. The foam polystyrene will expand when heated. The molds are brought into engagement with the expanded, contiguous, foam sheets. The sheets are pushed together into a female mold cavity provided on one of the molds via a plug assist provided on the other of the molds. Vacuum is then applied to another cavity provided in the male plug assist to withdraw a portion of one sheet to form sheet strengthening ribs therein. The male plug assist is constructed such that the width of the ribs formed is much greater than the width of the fuzed portions to minimize the area of the sheet subjected to "chill" by the molds.
Another object of the present invention is to provide twin sheet thermoforming apparatus and a method of forming a double wall vessel including the step of pushing portions of contiguous sheets into a female mold with a male plug assist having ribs and cavities therein to concurrently fuse border edge portions and to fuze opposed central portions of the sheets and to thereafter withdraw other opposed central portions.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art as the description thereof proceeds.